A variety of engine air/fuel control systems are known in which fuel delivered to the engine is adjusted in response to the output of one or more UEGO sensors, often to maintain an average air/fuel ratio at a stoichiometric value. Examples of such systems are disclosed in U.S. Pat. Nos. 5,255,512 and 5,282,360. Such systems may also include a fuel vapor recovery system wherein fuel vapors are purged from the fuel system into the engine's air/fuel intake. An example of such a system is disclosed in U.S. Pat. No. 5,048,493. Generally in these systems, an electronic controller calculates desired air/fuel levels over time based upon certain engine operating parameters and system measurements. One such system measurement is the oxygen content in the exhaust stream provided as feedback data by one or more UEGO sensors. Based on the calculated desirable air/fuel level, the electronic controller provides a control signal to the engine's fuel injectors to deliver a certain level of fuel to the engine cylinders. The control signal corresponds to a commanded or desirable air/fuel level.
A number of systematic errors are present in such systems that affect the accuracy of the air/fuel levels delivered to the engine cylinders. That is, the collective effects of a variety of systematic errors in the system cause the actual air/fuel levels delivered to the engine cylinders to vary from the calculated desirable air/fuel levels. These systematic errors may result from certain inaccuracies of the measurements derived from the UEGO sensor(s), airflow sensor(s) and other sensors in the system that provide feedback signals to the electronic controller. Also, a systematic fuel flow error resulting from variations in the level of fuel delivered by different fuel injectors in response to the same control signal may affect the accuracy of fuel delivery to the engine cylinders. Another type of systematic error results from variations in the composition of the fuel vapor and air mixture from the vapor recovery system. The collective effect of these various individual sources of error is considered the total system fuel error.
It is desirable for the system to monitor and correct for its systematic errors to achieve optimal air/fuel levels. However, even though the functional characteristics of certain system components under various operating conditions are predictable, until the present invention it has been difficult or impossible to correct for these systematic errors when using UEGO sensors because their respective individual contributions to the total system fuel error are undetectable. While it is generally known, for example, that variations in the internal gas diffusion rates from one UEGO sensor to another result in measurement errors that tend to vary linearly with the oxygen content of the exhaust gas, the inventor herein has recognized that this known operational characteristic can be used to correct for systematic UEGO sensor errors only if the UEGO errors can be apportioned from the other systematic errors that comprise the total system fuel error.